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I'm trying to call a class member variable from within its class, but I get a NameError: name '...' is not defined.

A similar situation is created by means of the following minimum working example:

from pprint import pprint class MyClass: _my_class_variable = {'key_0': 0, 'key_1': 1} _my_keys = _my_class_variable.keys() pprint(_my_class_variable) # WORKS! pprint([value for value in _my_class_variable.values()]) # WORKS! pprint([_my_class_variable[key] for key in _my_keys]) # DOES NOT WORK! pprint([_my_class_variable[key] for key in _my_class_variable.keys()]) # DOES NOT WORK!

which returns NameError: name '_my_class_variable' is not defined.

How is it possible that the first two pprint command work but not the last two pprint commands?

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Everything in the list comprehension is run in a separate scope (as a function, basically), except for the iterable used for the loop. So, on the lines that don't work, _my_class_variable is not defined.

One way to solve this would be to make sure that you pass _my_class_variable to an immediately executing lambda function so that it's available in the list comprehension's scope :

pprint((lambda _my_class_variable : [_my_class_variable[key] for key in _my_class_variable.keys()])(_my_class_variable)) # DOES WORK! # or pprint((lambda _my_class_variable=_my_class_variable : [_my_class_variable[key] for key in _my_class_variable.keys()])()) # DOES WORK!

Why does this work? List comprehensions get their own separate scope in Python 3 and are wrapped in a temporary function object and called immediately. If you were to create an explicit scope for the _my_class_variable, like in a function, its scope will be considered when resolving _my_class_variable.

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